Core Physics Topic 12 - Red Shift

 

 

Exploring the Universe

The exploration of space is called astronomy

 

Astronomy has NOTHING to do with astrology which deals with horoscopes (the daily drivel you get in some popular newspapers telling you that Venus will make it a good day for financial speculation).

 

Space is big.  Although we have sent men to the Moon and probes to the planets of the solar system, the distances are so massive that the probes have hardly got anywhere!  So most of our observations in space have come from using telescopes of different kinds.  These include:

X-rays and gamma rays are not easy to observe through a telescope.

 

Ground-based light telescopes have advantages:

There are disadvantages:

These problems can be overcome by flying the telescopes very high in the atmosphere in a large aeroplane.  This is expensive, and the quality of the images depends on the skill of the pilot.

 

NASA

 

Some telescopes are sent into orbit as satellites, giving images of stunning quality.  The most famous is the Hubble Space Telescope.  Space telescopes give astronomers the opportunity to observe in the infra-red, UV, X-ray, and gamma ray regions.  The table shows some of the objects that can be seen with different wavelengths of electromagnetic radiation.

 

Wavelength

Object in Space

Radio

Pulsars

Microwave

Background radiation

Far infra red

Protostars and planets

Infra red

Red giants

Visible

Stars

Ultra violet

Hot stars, quasars

X-rays

Neutron Stars

Gamma rays

Neutron stars, black holes

 

Space Travel

Journeys into space are expensive.  The vast majority have involved unmanned probes.  The furthest probes are the Voyager 1 and 2 spacecraft that were launched in the mid nineteen seventies, when your parents were born.  They have a mass of 773 kg and an output power of 420 W, although this would now be considerably less now.  It is powered by solar cells on panels.  The power on the panels would be a fraction of what it was when it was close to the Earth.  Voyager 1 has passed through the outer reach of the Solar System, while Voyager 2 is still just in the outer reaches, about  18 109 km.  You can see more about the Voyager mission here:

 

https://www.space.com/17205-voyager-spacecraft.html

 

https://voyager.jpl.nasa.gov/mission/status/

 

Both vehicles are travelling at about 60 000 m s-1. which means that it will take many years to get anywhere.

 

Many probes are powered by chemical motors, but ion drives have also been used.  In space, where there is almost no friction, the motors only need to be fired when the probe is accelerating, or changing direction.  In tests, ion drives use much less propellant than chemical rocket motors.  In one case, a xenon ion drive used about 800 kg of xenon, while the equivalent rocket motor used 10 000 kg of fuel.  The forward thrust of an ion drive is less than 1 N, about the thrust of a toy plane.  However since there is no friction in space, the probe will accelerate, and achieve high speeds if the ion drive is left on for long enough.

 

Another way of accelerating probes is to use gravity from planets and moons to catapult the probe to a higher speed.  This requires precision timing to send the commands from an Earth based station to the satellite.  Small motors to manoeuvre the probe have to be fired at exactly the right time.  While the commands travel at the speed of light, there is a definite time delay which has to be taken into account.  Get the time wrong, and the probe goes off in the wrong direction.  (This happened when all the calculations were done in kilometres, but the American engineers assumed they were done in miles.)

 

 

Humans have travelled to the moon, 400 000 km.  There are plans to send humans to Mars.  However there are many problems to overcome before such journeys could be considered as viable, among which are:

 

Getting from Earth to Space and back again is a risky business:

 

 

Doppler Shift

You may well have been on a street when an ambulance has gone past with its siren blaring.  The note of the siren has a high pitch as it approaches, and the pitch goes down as it moves away.  This change in frequency is called the Doppler Effect.  The picture shows the idea with a train.

 

Source not known

The Doppler effect also works with light as we will see next.

 

About a hundred years ago, astronomers observed an interesting result.  If you analyse elements with an instrument called a spectroscope, you find that each element has a characteristic pattern of coloured lines.  If you put the coloured lines against a spectrum of visible light, the coloured lines show up as black lines, but the pattern is the same.  Astronomers found this to be a good way of working out what chemicals there were in stars.

 

They found that when they analysed elements in distant stars, the patterns were there, but moved over towards the red end of the spectrum (longer wavelength). 

 

 

You can see how the pattern is the same, but the colours of the lines have changed.  For example the blue line has become green; its wavelength has increased.

 

This could be explained by the Doppler Effect.  The longer wavelength (lower frequency) is called Red-Shift, and shows that the star is moving away from us.  The same effect is seen with radio waves or microwaves.

 

The further the star is away from us, the more the red-shift, which means the faster the star is travelling away from us.

 

Question 1

What would you see if a star or galaxy was coming towards us? 

Answer

 

 

How the Universe Began

Astronomers have observed that stars and galaxies are moving away from us. Therefore they believe that everything was once contained in one place. All the matter in the universe was contained as energy in the space of a pinhead.

 

The energy was released in a titanic explosion called the Big Bang. (This term was first used in a radio broadcast about fifty years ago by the then Astronomer Royal, Sir Fred Hoyle, who didn't believe a word of it and was pretty sarcastic about the theory.)   The temperature was billions of Kelvin. Then within seconds, some of the energy turned into matter, making electrons, protons, and neutrons. These in turn started to make the simple atoms like Hydrogen (1 proton and 1 electron).

 

In the massively high temperatures of the Big Bang (1032 K), a whole zoo of particles was made, for example:

Particle Physicists are using massive machines to produce the conditions after the Big Bang. They have got within 10-30 s after the Big Bang.

Conditions cooled rapidly after the Big Bang.  After 15 000 million years, the temperature of Outer Space is 3 Kelvin (-270 oC), not very warm.

 

 

NASA

The Big Bang is thought to have occurred about 18 000 million years ago.

 

What is the evidence for this?

NASA

 

Light travels at 300 million m/s. It takes 8 minutes for the light of the Sun to get to us. The nearest star other than the Sun is 4 light years away. A light year is the distance that light travels in 1 year, which is about 1016 m. So the light that left that star did so in 2004. If the star went out now, we wouldn't know about it for 4 years.

 

Light from distant galaxies takes thousands of millions of years to get to us. Therefore we can say that what we see now is what the galaxies were like all that time ago. Many of them might well not exist now.

 

Cosmic Microwave Background Radiation (CMBR) has also been discovered, which is considered to be the afterglow of the Big Bang.  This is microwave radiation out at the very edges of the universe, giving us an insight into what the universe was like 400 000 years after the Big Bang.  It explains why the Universe has a temperature of 2 K, not 0 K.

 

 

Material was thrown out as the Universe expanded, and in places came together under the influence of gravity to form galaxies, stars, and planets.  It is a mind-boggling thought that the light reaching us from the most distant galaxies left those galaxies not long after the big bang.  There are many thousands of millions of stars.  Latest evidence suggests that most have planets, and some of these are thought to have conditions that are ideal for life.  Who knows; there may be life.

 

Many astronomers believe in other theories, e.g. the Steady State theory.  However the Big Bang theory is the only one that can explain the evidence.

 

Question 2

Use this picture to answer the questions

(a) How long after the Big Bang were there microwaves?  What is the evidence?

(b) What does the Spitzer telescope detect?

(c) How long ago did that radiation occur?

(d) How old is the Universe now?

Answer

 

The fate of the Universe is of interest to scientists. There are three possibilities:

  1. The force of gravity may overcome the expansion, which will slow down. Then it will stop, and all the galaxies will come together again. This is the opposite to the Big Bang, and is called the Big Crunch. It's like a cosmic bungee-jump.

  2. The force of gravity is not strong enough to overcome the expansion, which will continue. The Universe will be a cold, lonely, and bleak place.

  3. The force of gravity and the expansion will balance out, leaving the Universe in a stable state.

It all depends on how much material there is in the Universe. We can see a lot of it, but there is thought to be even more that we can't see. It is called dark matter. Until scientists know exactly how much there is, which one of the three will be the fate cannot be known for sure.

What is certain is that it will happen eons after we are all long dead, gone, and forgotten.

 

 

Summary

  • Telescopes are used to observe the universe;

  • Satellites and probes are also used;

  • Ground based telescopes are limited in use;

  • Space telescopes give very clear images;

  • Telescopes can be made to use any part of the electromagnetic spectrum;

  • Most astronomers believe that the Universe was formed by the Big Bang.